Because of their chemical inertia, ion-exchange membranes partially or completely fluorinated are usually chosen for alkali-chloride processes or fuel cells consuming hydrogen or methanol. Such membranes are commercially available under trade names like Nafion™, Flemion™, Dow™. Other similar membranes are proposed by Ballard Inc. in application WO 97/25369 that describes copolymers of tetrafluoroethylene and perfluorovinylethers or trifluorovinylstyrene. The active monomers from which these copolymers are obtained bear chemical functions that are the precursors of ionic groups of the sulfonate or carboxylate type. Example of such precursors are:
wherein                X is F, Cl or CF3;        n is 0 to 10 inclusively; and        p is 1 or 2.        
Aromatic polymers of the polyimide or sulfonated polyether sulfone type have also been considered, for example:

Once obtained, the copolymer containing the above precursors is molded, for example in the form of sheets, and converted into an ionic form through hydrolysis, to give species of the sulfonate or carboxylate type. The cation associated to the sulfonate and carboxylate anion include the proton, all alkali metal cation (Li+, Na+, K+); an alkaline-earth metal cation (Mg2+, Ca2+, Ba2+); a transition metal cation (Zn2+, Cu2+); Al3+; Fe3+; a rare earth cation (Sc3+, Y3+, La3+); an organic cation of the “onium” type, such as oxonium, ammonium, pyridinium, guanidinium, amidinium, sulfonium, phosphonium, these organic cations being optionally substituted by one or mole organic radicals; an organometallic cation such as metallocenium, arene-metallocenium, alkylsilyl, alkylgermanyl or alkyltin.
Such membranes suffer from many important disadvantages.    A) Although the copolymers forming the membrane are insoluble in their ionic form, the membrane does not have a good dimensional stability and swells significantly in water or polar solvents. These copolymers form inverted micellia only when heated at high temperatures in a specific mixture water-alcohol that, after evaporation, allows the production of a film. However, this film regenerated in the solid form does not have good mechanical properties.    B) Tetrafluoroethylene (TFE) is a hazardous product to handle, because its polymerisation is performed under pressure and can cause uncontrolled reactions, particularly in the presence of oxygen. Because of the difference of boiling points between the two monomers forming the copolymer, as well as their polarity difference, it is difficult to obtain a statistical copolymer corresponding to the addition rate of each monomer.    C) The ionic groups in high concentration on the chain have a tendency to cause solubilisation of the copolymer. To prevent this phenomenon, the concentration of ionic groups is kept fairly low by adding an important molar fraction of TFE monomers and/or by increasing the secondary chains length (n>1), with the end result that the concentration of the exchangeable ion groups are less than 1 milliequivalent per gram. Consequently, the conductivity is relatively low and highly sensitive to the water content of the membrane, particularly when the latter is acidified for applications in a fuel cell.    D) The penetration of methanol and oxygen through the membrane is high, because the perfluorocarbonated portion of the polymer allows an easy diffusion of the molecular species, which will chemically react at the opposite electrode and cause a loss of faradic efficiency, mainly in methanol fuel cells.
Non-fluorinated systems like sulfonated polyimides or sulfonated polyether sulfones have the same drawbacks because one must compromise between the charged density, and thus the conductivity, and the solubility or excessive swelling.